PSEUDOCHLAMYS PATELLA. 131 



body to the interior of the test, the latter appearing to 

 be fully occupied, when the discoid figure is examined, 

 except a narrow hyaline margin. Usually the animal 

 begins an active movement by raising the margin of 

 the test on the opposite side ; the movement will be 

 continued until the opposite margins approach and 

 the figure becomes roughly-speaking semi-circular ; or 

 the margins will overlap, and the animal will remain 

 thus curiously rolled together for some time, its 

 general form being comparable to that of the chrysalis 

 of a. lepidopterous insect. 



The discoid form admits of the endoplasm being 

 readily studied. Four or more contractile vacuoles 

 may usually be seen, pulsating languidly, on the outer 

 margin of the plasma-body ; and a central nucleus is 

 also visible, though not conspicuous. 



The brown colour of the test and its rigidity are dis- 

 tinctive of old individuals. Empty tests may at any 

 time be met with where the species is abundant ; they 

 are seen to vary little, if at all, in size or structure. 

 They differ widely in appearance from the tests of the 

 Arcellae, being very much smaller, simple, and concave, 

 usually of a deep brown, and sometimes punctated 

 about the centre, the colour gradually shading off 

 towards the margin, which is rarely found inverted. 



Although undoubtedly distinct, the affinities of 

 Pseudoclilamys are with Arcella. We have found the 

 organism most abundant in spring, amongst floating 

 Hypna in ponds. Later in the year it seems to 

 disappear. 



Genus 17. CENTROPYXIS Stein, 1859. 



Arcella (pars) EHRENBERG in Abh. K. Akad. Wiss. Berlin, 



1830 (1832), p. 40. 



Arcellina CANTON in Ann. Nat. Hist. (1) IX (1842), p. 362. 

 Bifflugia (pars) PERTY in Mitth. nat. Ges. Bern, 1849, p. 45. 

 Centropyxis STEIN in Abh. K. Bohm. Ges. Wiss. X (1859), 



Berichte, p. 43. 

 Echinopyxis CLAPAREDE and LACHMANN Etudes Inf. et. Ehiz. 



2 (1859), p. 447. 



